Electrode for electric discharge lamps



Qbt. 27, 1953 H. H. HOMER ETAL 2,657,325

ELECTRODE FOR ELECTRIC DISCHARGE LAMPS Filed Dec; 23, 1950 CASE @154 0- I Tl/A/Gfl'E/V -0 +5 I fiy. us/my 01/0/2150 5:10 H CASE 15 4 0 Ba HFAV/l) v CASE 0- 0- m Ba+ 02 IINVIENTORS:

HORACE H.- HOMER ERWIN F. LOWRY ATTORNEY Patented Oct. 27, 1953 2,657,325 ELECTRODEFOR ELECTRIC DISCHARGE LAMPS Horace H. Homer, Arlington, and Erwin F. Lowry, Lynn, Mass., assignors to Sylvania Electric Products 1110., Salem, Mass., a corporation of Massachusetts Application December 23, 1950, Serial No. 202,531

4 Claims.

This invention relates to electric gaseous discharge lamps, such as fluorescent lamps, and particularly to electrodes therefor.

Such electrodes generally have a metal base member coated or filled with an electron-emitting material such as the alkaline earth oxides. The metal member is usually in the form of a wire wound into a small-diameter coil, the latter being itself wound into a large-diameter coil to form a coiled coil. The small-diameter coil is called the minor coil, the large-diameter coil the major coil. An electron emissive material is used as a filling for the minor coil only, the interior of the major coil being otherwise free of such materiaL'as shown for example in U. S. Patent 2,258,158.

The electron-emitting material gradually disintegrates, causing blackening on the glass envelope in the vicinity of the electrodes. This disintegration produces two chief types of discoloration, anode spots, which appear on the glass beside the electrodes, and end bands, which appear a short distance in front of the electrodes.

The anode spot, so called because it appears only at the anode when the device is operated on direct current, usually occurs early in the life of the lamp. The end-band, which appears only at the cathode when the device is operated on direct current, usually occurs later in life and is chiefly a yellow-brown oxide of mercury, which blackens by absorbing mercury.

The purpose of our invention is to reduce the rate of disintegration of the cathode. thereby prolonging the life of the lamp and retarding or suppressing the formation of discoloring spots or hands. This is achieved by providing, at the interface between the metal of the base and the electron-emitting material it carries, a monomolecular layer of oxide of said metal, presumably in the form of a polar compound.

Such a layer is a feature of our device, other features and advantages of which will be apparent from the following specification, taken in conjunction with the accompanying drawings in which:

, Fig. 1 is a side view, partly in S ction, of a lamp according to the invention;

Fig. 2 is a schematic diagram, showing the combination between the atoms when the surface of the electrode is initially clean, that is free of oxide;

Fig. 3 is a similar type of diagram, showing the result when the metal has a monomolecular layer of oxide;

Fig. 4' is another such diagram, for the case where the metal is initially heavily oxidized; and

Fig. 5 is a view of the cathode construction.

In Fig. 1, the tubular glass envelope I is coated on its interior surface with a phosphor, by methods customary in the art. The envelope is sealed at each end to a stem I2 having the usual glass press 3 through which lead-in wires 4 and 5 extend to support and make electrical contact with opposite ends of the tungsten wire 6, which is doubly-coiled as in the previously-mentioned patent and as shown in Fig. 5 herein, the electron-emitting material being in the form of one or more of the alkaline earth oxides and confined to the inside of the small-diameter coil of the first or minor coiling.

,The lead-in wires 4, 5, respectively, extend through the stem press 3, to the contact pins 1, 8 of base 9, affixed to the end of the lamp by the cement it. There is an identical electrode 6 and base 6 at each end. of the lamp, to permit operation on alternating current in the usual circuits.

The sealed envelope I contains a filling of inert gas such as argon at a pressure of about 3 millimeters of mercury, and also contains a drop of mercury to produce mercury vapor. The lamp is normally operated at a current and voltage proper to produce a mercury vapor pressure in the vicinity of 10 microns.

We have found a mixture of about 48% barium oxide, 16% calcium oxide and 42% strontium oxide, the percentages taken by moles, to be very effective as an electron-emitting material, although other proportions of the components can be used, as shown for example in United States Patent Number 2,547,869, granted April 3, 1951, to Gerald W. Keilholtz and Charles W. Jerome. The electron-emissive materials were applied as a suspension of the corresponding carbonates in an 18-second viscosity solution of NOD-second nitrocellulose in amyl acetate. The 18 seconds viscosity was measured in a Stormer viscometer. About 100 grams of carbonates were suspended in cc. of the nitrocellulose solution.

The doubly-coiled wire of the electrode is dipped into the above suspension, and any excess solution over that needed to fill the primary coil is blown away by an air stream. The carbonates are later reduced to the oxides by passing a heating current through the wire at an intermediate stage of the exhaust process for the lamp.

The monomolecular oxide-coating of the metal of the base member of the cathode can be applied in various ways. It can be applied by directly oxidizing the metal, usually tungsten, before applying the carbonate coating, and then exhausting the lamp rapidly while the alkaline earth carbonates. are broken dow 29 the alkaline earth oxides by heating, to prevent any further oxidation of the metal by the carbon dioxide given on.

Or the wire can be initially free from appreciable oxidation, and can be oxidized by allowing the carbon dioxide, produced at breakdown of the carbonates, to oxidize the tungsten or other metal sumciently, and be itself reduced. In this case the exhaust at the time of breakdown can be slower. The first method is more adaptable to machines with high speed exhaust schedules, the latter to those with slower schedules. In either case, the time and intensity of oxidation should be adjusted to give a monomolecular layer of the metal oxide.

It is often noticed in practice that discoloration appears at one end of the lamp much more strongly than at the other. With a poor exhaust system, this is due to the fact that the end in the vicinity of the exhaust tube, the latter being present at only one end of the lamp, is exhausted more fully than the end of the tube farthest from the exhaust tube. The diiference is due to the great length of fluorescent tubes.

However, with the good modern exhaust systems, the situation is reversed. The discoloration is generally greater at the exhaust tube end of the tube, because as the entire lamp gets thoroughly exhausted the gases from the cathode at the far end of the lamp are carried around the cathode at the near end of the tube, thereby increasing the concentration of carbon dioxide and the like around the near electrode. Neal and far are used with reference to the distance from the exhaust tube.

This difference in discoloration at different ends of the lamp can be reduced or eliminated by having an initial monomolecular layer of oxide on the metal of the far cathode, and no oxide, or only a partial layer, on the near electrode. Then the carbon dioxide can be exhausted away quickly enough from the far electrode to prevent excessive oxidation and the increase in carbon dioxide concentration at the near electrode will enable the oxidation of the near electrode to the proper extent.

If the increased carbon dioxide content were not of itself sufficient to achieve proper oxidation, the temperature of the near electrode could be raised by the passage of current through it, but if the oxidation tended to be too great the temperatures of the near electrode could be made lower than that of the far electrode.

In fact, even if the two electrodes are of the same initial oxidation status, the near electrode should be operated at a lower temperature than the far cathode, in order to equalize the final oxidation.

The lamp should, of course, have a preliminary evacuation to remove the air, before the electrodes are heated at all.

In some cases, the small amount of nitrocellulose in the suspension may be deleterious, as leaving a small carbon residue which may tend to reduce the oxide and then be lost as carbon dioxide. In this case, a high-viscosity methacrylate polymer may be used in the suspension instead of nitrocellulose, the polymer being dissolved in xylol, for example. sobutyl methacrylate can be used, for example. This will go ofi on heating without any carbon residue and as a non-oxidizing gas.

Figure 2 shows the result of having less than a monomolecular layer of oxide on the metal. In this case, the clean tungsten surface reacts with the barium oxide to form at the interface a polar compound of barium-on-oxygen-on-tungsten, with the formation of free barium. When the electrode is heated, the latter is evaporated off to form an anode spot on the glass envelope where it condenses.

If the amount of metal oxide is monomolecular as indicated in Figure 3 then the same polar compound is formed as before, but there is no reduction of barium oxide by the tungsten, therefore no free barium to be evaporated.

However, if the tungsten is oxidized more heavily than a monomolecular layer, the formation of the polar compound will release oxygen from the barium oxide, and this oxygen will combine with the mercury or other metal vapor in the lamp to form end bands.

The threshold for band formation appears to be in the neighborhood of an oxidation for 15 minutes at 300 F. in air, which corresponds to about 5 x 10- grams of oxide per square centimeter of tungsten.

What we claim is:

1. An electric gaseous discharge lamp comprising a sealed light-transmissive envelope, an inert gas at low pressure therein, a quantity of mercury therein, a tungsten electrode at each end of said envelope, an alkaline earth oxide carried by said electrode, and a monomolecular layer of oxide of said tungsten at the interface between the tungsten and the oxide.

2. An electric gaseous discharge lamp comprising a sealed light-transmissive envelope, an inert gas at low pressure therein, a quantity of mercury therein, a coiled-coil electrode of tungsten wire at each end of said envelope, an alkaline earth oxide filling within the turns of only the minor coil of said coiled-coil, and a monomolecular layer of oxide of said tungsten at the interface between the tungsten wire and the alkaline earth oxide.

3. An electric gaseous discharge lamp consisting essentially of a sealed light-transmissive envelope, an inert gas at low pressure therein, a quantity of mercury therein, a tungsten electrode at each end of said envelope, an alkaline earth oxide carried by said electrode, and a monomolecular layer of oxide of said tungsten at the interface between the tungsten and the oxide.

l. An electric gaseous discharge lamp consisting essentially of a sealed light-transmissive envelope, an inert gas at low pressure therein, a quantity of mercury therein, a coiled-coil electrode of tungsten wire at each end of said en- .velope, an alkaline earth oxide filling within the turns of only the minor coil of said coiled-coil, and a monomolecular layer of oxide of said tungsten at the interface between the tungsten wire and the alkaline earth oxide.

HORACE H. HOMER. ERWIN F. LOWRY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,699,639 Gessel Jan. 22, 1929 2,08%,911 Kayatt June 22, 1937 2,258,158 Lowry Oct. 7, 1941 2,275,886 Bondley Mar. 10, 1942 FOREIGN PATENTS Number Country Date 313,151 Great Britain June 12, 1930 337,971 Great Britain Nov. 13, 1930 

1. AN ELECTRIC GASEOUS DISCHARGE LAMP COMPRISING A SEALED LIGHT-TRANSMISSIVE ENVELOPE, AN INERT GAS AT LOW PRESSURE THEREIN, A QUANTITY OF MERCURY THEREIN, A TUNGSTEN ELECTRODE AT EACH END OF SAID ENVELOPE, AN ALKALINE EARTH OXIDE CARRIED BY SAID ELECTRODE, AND A MONOMOLECULAR LAYER OF OXIDE OF SAID TUNGSTEN AT THE INTERFACE BETWEEN THE TUNGSTEN AND THE OXIDE. 